Ask astronomers about dark matter and one of the things they talk about is that this invisible and mysterious “stuff” pervades the universe. In particular, it exists in the halos surrounding most galaxies.
The mass of the halo exerts a strong gravitational influence on the galaxy itself, as well as others in the vicinity. That’s pretty much the standard view of dark matter and its influence on galaxies.
However, there are problems with the idea of these halos. Apparently, there are oddly shaped dwarf galaxies that seem to have no halos. How is it possible ? Do they represent a challenge induced by the observation of the dominant ideas on dark matter halos?
Find Disturbed Dwarf Galaxies
In the so-called “standard model” of cosmology, shells or halos of dark matter shield galaxies from the gravitational influence of nearby galactic neighbors.
However, when astronomers from the University of Bonn and Saint Andrews in Scotland looked into the nearby Fornax cluster, which lies about 62 million light-years away from us, they saw something strange. .
It contains a number of dwarf galaxies with distorted and disturbed shapes. It’s strange, especially if they are to be surrounded by halos of dark matter.
Let’s take a look at dwarf galaxies. They are small and weak and are usually found in clusters of galaxies or near much larger companions. The Milky Way galaxy is surrounded by a coterie of dwarf galaxies.
It’s actually cannibalizing species such as the Sagittarius dwarf spheroid. Interestingly, recent studies show that at least one of the dwarf galaxies near ours, an older one called Tucana II, has a surprisingly massive dark matter halo.
So what’s different at Fornax?
There, the dwarf galaxies could be “disturbed” by the gravitational tides of the cluster’s larger neighbors. Tides occur when one body’s gravity pulls differently on different parts of another body. These are similar to the tides on Earth when the Moon pulls harder on the side of the Earth facing it.
The distorted shapes of the dwarf galaxies seen by the team point to a problem with our understanding of dark matter.
“Such perturbations in Fornax dwarfs are not expected according to the Standard Model,” said Pavel Kroupa, a professor at the University of Bonn and Charles University in Prague.
“Indeed, according to this model, the dark matter halos of these dwarfs should in part protect them from the tides raised by the cluster.”
Explain distorted dwarf galaxies
Kroupa and Ph.D. student Elena Ascencio analyzed sightings of disturbed dwarves in Fornax. They wanted to understand the extent of the gravitational distortions that these galaxies show and what causes them.
Expected levels of distortion depend on several factors. One is the internal characteristics of the dwarf galaxy. Moreover, their distance from the center of the cluster is important. This is where the gravitational influences are much stronger.
Typically, galaxies that are large in size but few stars could be easily perturbed by strong gravitational tides. The same is true for galaxies closer to the core of the cluster.
Team members compared what they saw in the cluster with observations made by the VLT Survey Telescope at the European Southern Observatory. Asencio pointed out that what they found seems to indicate problems with the standard model.
“The comparison showed that, if one wants to explain the observations in the Standard Model,” she said, “the Fornax dwarfs should already be destroyed by gravity from the center of the cluster even when the tides it lifts on a dwarf are sixty-four times weaker than the dwarf’s own gravity.”
Not only is this counterintuitive, she says, but it also contradicts previous studies. The team also discovered that the force needed to disrupt a dwarf galaxy is about the same as its own gravity.
What does this mean for the standard model?
The research team points out that it is difficult to explain these perturbed and disturbed forms of Fornax dwarf galaxies if they are surrounded by dark matter. In other words, they should not be distorted if they have halos.
Yet they are there with troubled forms. This means that there are no dark matter halos around these galaxies.
Obviously, if what the astronomers have found is confirmed, then the Standard Model needs some tweaking. And, there is at least one alternative explanation for the strange shapes of galaxies. This is called the MOND model (short for Modified Newtonian Dynamics).
This suggests that Newton’s law of universal gravitation should be modified to account for the observed properties of galaxies. This could be applied to explain why distorted galaxies look like this.
According to research team member Hongsheng Zhao from the University of Saint Andrews, finding perturbed dwarfs without dark matter halos is a major challenge to the current view.
He states that galaxies have halos. It seems that this is not the case for all, he underlines.
“Our results have major implications for fundamental physics,” he said. “We expect to find more perturbed dwarfs in other clusters, a prediction that other teams should verify.”